Magnetic keys having a plurality of magnet layers with holes

ABSTRACT

Magnetic keys having a plurality of magnetic layers having holes are disclosed. The location and orientation of the holes are controlled to generate magnetic fields that are of sufficient strength to be reliably read and sufficient complexity to be difficult to counterfeit. The magnetic keys are located on imaging-device supply items along with non-volatile memory devices containing measurements of the magnetic fields that are digitally signed. These supply items are difficult to counterfeit. Other devices are disclosed.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.15/373,967 filed Dec. 9, 2016 titled MAGNETIC KEYS HAVING A PLURALITY OFMAGNETIC PLATES.

BACKGROUND 1. Field of the Disclosure

The present disclosure relates generally to anti-counterfeit systems andmore particularly to magnetic keys on supply items.

2. Description of the Related Art

Counterfeit printer supplies, such as toner bottles, are a problem forconsumers. Counterfeit supplies may perform poorly and may damageprinters. Printer manufacturers use authentication systems to detercounterfeiters. Physical unclonable functions (PUF) are a type ofauthentication system that implements a physical one-way function.Ideally, a PUF cannot be identically replicated and thus is difficult tocounterfeit. Thus, it is advantageous to maximize the difficulty ofreplicating a PUP to deter counterfeiters.

PUFs have been proposed that contain random distributions of magneticparticles in a non-magnetic substrate. Since the distribution is random,it is difficult to ensure that the generated magnetic field will havesufficient strength to be reliably read by low-cost magnetic fieldsensors. Also, it is difficult to ensure that the generated magneticfield will be sufficiently complex to be difficult to counterfeit. Whatis needed is a magnetic key that overcomes these deficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification, illustrate several aspects of the present disclosure, andtogether with the description serve to explain the principles of thepresent disclosure.

FIG. 1 is a block diagram of an imaging system including an imageforming device according to one example embodiment.

FIG. 2 is a block diagram of a toner bottle having a magnetic key.

FIG. 3 is a graph of intensity of a magnetic field along an outersurface of a magnetic key.

FIG. 4 is an example of generating a digital signature from an array ofnumbers.

FIG. 5 is a top view of a magnetic plate.

FIG. 6 is a side view of a magnetic plate.

FIG. 7 is a top view of a magnetic plate.

FIG. 8 is a side view of a plurality of magnetic plates.

FIG. 9 is a top view of a plurality of magnetic plates.

FIG. 10 is a top view of a plurality of magnetic plates.

FIG. 11 is a top view of a plurality of magnetic plates.

FIG. 12 is a side view of a magnetic key having a plurality of magneticplates.

FIG. 13 is a side view of a magnetic key having a plurality of magneticplates.

FIG. 14 is a top view of a magnet having a plurality of holes.

FIG. 15 is a side view of a magnetic key.

FIG. 16 is a top view of a magnetic key having a plurality of holes.

FIG. 17 is a top view of a magnetic key having a plurality of holes.

FIG. 18 is a top view of a magnetic key having a plurality of holes.

DETAILED DESCRIPTION

in the following description, reference is made to the accompanyingdrawings where like numerals represent like elements. The embodimentsare described in sufficient detail to enable those skilled in the art topractice the present disclosure. It is to be understood that otherembodiments may be utilized and that process, electrical, and mechanicalchanges, etc., may be made without departing from the scope of thepresent disclosure. Examples merely typify possible variations. Portionsand features of some embodiments may be included in or substituted forthose of others. The following description, therefore, is not to betaken in a limiting sense and the scope of the present disclosure isdefined only by the appended claims and their equivalents.

Referring to the drawings and particularly to FIG. 1, there is shown ablock diagram depiction of an imaging system 50 according to one exampleembodiment. Imaging system 50 includes an image forming device 100 and acomputer 60. Image forming device 100 communicates with computer 60 viaa communications link 70. As used herein, the term “communications link”generally refers to any structure that facilitates electroniccommunication between multiple components and may operate using wired orwireless technology and may include communications over the Internet.

In the example embodiment shown in FIG. 1, image forming device 100 is amultifunction device (sometimes referred to as an all-in-one (AIO)device) that includes a controller 102, a user interface 104, a printengine 110, a laser scan unit (LSU) 112, one or more toner bottles orcartridges 200, one or more imaging units 300, a fuser 120, a media feedsystem 130 and media input tray 140, and a scanner system 150. Imageforming device 100 may communicate with computer 60 via a standardcommunication protocol, such as, for example, universal serial bus(USB), Ethernet or IEEE 802.xx. Image forming device 100 may be, forexample, an electrophotographic printer/copier including an integratedscanner system 150 or a standalone electrophotographic printer. Tonerbottles 200 and fusers 120 are supply items that may be userreplaceable.

Controller 102 includes a processor unit and associated memory 103 andmay be formed as one or more Application Specific Integrated. Circuits(ASICs). Memory 103 may be any volatile or non-volatile memory orcombination thereof such as, for example, random access memory (RAM),read only memory (ROM), flash memory and/or non-volatile RAM (NVRAM).Alternatively, memory 103 may be in the form of a separate electronicmemory (e.g., RAM, ROM, and/or NVRAM), a hard drive, a CD or DVD drive,or any memory device convenient for use with controller 102. Controller102 may be, for example, a combined printer and scanner controller.

In the example embodiment illustrated, controller 102 communicates withprint engine 110 via a communications link 160. Controller 102communicates with imaging unit(s) 300 and processing circuitry 301 oneach imaging unit 300 via communications link(s) 161. Controller 102communicates with toner cartridge(s) 200 and non-volatile memory 201 oneach toner cartridge 200 via communications link(s) 162. Controller 102communicates with fuser 120 and processing circuitry 121 thereon via acommunications link 163. Controller 102 communicates with media feedsystem 130 via a communications link 164. Controller 102 communicateswith scanner system 150 via a communications link 165. User interface104 is communicatively coupled to controller 102 via a communicationslink 166. Processing circuitry 121 and 301 may include a processor andassociated memory such as RAM, ROM, and/or non-volatile memory and mayprovide authentication functions, safety and operational interlocks,operating parameters and usage information related to fuser 120, tonercartridge(s) 200 and imaging unit(s) 300, respectively. Controller 102processes print and scan data and operates print engine 110 duringprinting and scanner system 150 during scanning.

Computer 60, which is optional, may be, for example, a personalcomputer, including memory 62, such as RAM, ROM, and/or NVRAM, an inputdevice 64, such as a keyboard and/or a mouse, and a display monitor 66.Computer 60 also includes a processor, input/output (I/O) interfaces,and may include at least one mass data storage device, such as a harddrive, a CD-ROM and/or a DVD unit (not shown). Computer 60 may also be adevice capable of communicating with image forming device 100 other thana personal computer such as, for example, a tablet computer, asmartphone, or other electronic device.

In the example embodiment illustrated, computer 60 includes in itsmemory a software program including program instructions that functionas an imaging driver 68, e.g., printer/scanner driver software, forimage forming device 100. Imaging driver 68 is in communication withcontroller 102 of image forming device 100 via communications link 70.Imaging driver 68 facilitates communication between image forming device100 and computer 60. One aspect of imaging driver 68 may be, forexample, to provide formatted print data to image forming device 100,and more particularly to print engine 110, to print an image. Anotheraspect of imaging driver 68 may be, for example, to facilitate thecollection of scanned data from scanner system 150.

In some circumstances, it may be desirable to operate image formingdevice 100 in a standalone mode. In the standalone mode, image formingdevice 100 is capable of functioning without computer 60. Accordingly,all or a portion of imaging driver 68, or a similar driver, may belocated in controller 102 of image forming device 100 so as toaccommodate printing and/or scanning functionality when operating in thestandalone mode.

Several components of the image forming device 100 are user replaceablee.g. toner cartridge 200, fuser 120, and imaging unit 300. It isadvantageous to prevent counterfeiting these user replaceablecomponents. A magnetic key 202 may be located on the toner cartridge 200to prevent counterfeiting as described below. A magnetic field reader203 may be integrated into the image forming device 100 to verify theauthenticity of the magnetic key 202. The magnetic field reader 203 mayinclude a magnetic field sensor attached to a linear-translation carrierto read a section of the magnetic key 202. Data related to the magnetickey 202 may reside in non-volatile memory 201.

FIG. 2 shows a side view of the toner bottle 200. The non-volatilememory 201 and magnetic key 202 are located on a body 210. The bodycontains toner that is consumed during the imaging process. The magnetickey 202 is elongate and contains magnetic plates as described below. Thenon-volatile memory 201 contains an array of numbers corresponding tothe intensity of the magnetic field above a first outer surface ofmagnetic key 202, e.g. top surface, side surface, bottom surface, etc.,at a plurality of locations along the length of the magnetic key e.g.along a linear path at evenly spaced intervals, at irregularly spacedintervals, etc. The array of numbers may also contain numberscorresponding to the intensity of the magnetic field at a secondplurality of locations along a second outer surface of the magnetic keythat is, for example, opposite the first surface, to make the magnetickey more difficult to counterfeit. The non-volatile memory 201 islocated on a printed circuit board 210 having a row of contact pads 212for making electrical connection to the image forming device 100.

FIG. 3 shows a graph 310 of the intensity 312 of an example magneticfield for one possible direction along a path along the outer surface ofthe magnetic key 202. An array of numbers 314 corresponds to themagnetic field intensity measured at regular intervals along the path,as shown by dotted lines 316 on the graph. Preferably, the array ofnumbers 314 are integers to simplify processing. Alternatively, thearray of numbers may be, for example, floating point. The numbers inFIG. 3 and FIG. 4 are in hexadecimal format in this example, themagnetic field intensity is always positive. Alternatively, the magneticfield intensity may be always negative, may alternate between positiveand negative, etc. Thus, the array of numbers 314 may contain positiveand negative numbers. The array of numbers 314 may, for example, containmeasurements of the magnetic field measured orthogonal to the outersurface. The array of numbers 314 may contain measurements of themagnetic field parallel to the outer surface. Preferably, the array ofnumbers contains measurements of the magnetic field along at least twoorthogonal directions at each location to make it more difficult tocounterfeit the magnetic key 202. Low cost magnetic field sensors areavailable that measure along multiple orthogonal directions, e.g. alongthree orthogonal directions.

FIG. 4 shows an example of generating a digital signature from the arrayof numbers 314. Other algorithms for generating a digital signature areknown in the art. The digital signature is used by the controller 102 toverify that the magnetic-key data in the non-volatile memory isauthentic. The toner bottle's serial number 410 and the array of numbers314 are combined to form a message 412. Preferably, the message isencrypted. Alternatively, the message may be unencrypted. For thisexample, AES-CBC is used (see, to for example, RFC3602 “The AES-CBCCipher Algorithm and Its Use with IPsec” published by The InternetSociety (2003), and NIST (National Institute of Standards) documentsFIPS-197 (for AES) and to SP800-38A (for CBC)). The AES key 414 and CBCInitialization Vector (IV) 416 are used as is known in the art togenerate the encrypted message 418. In this example, to sign theencrypted message 418 first the message is hashed then the hash isencrypted with the private key 420 of an asymmetric key pair thatincludes a public key 422. This example uses the SHA-512 hashingalgorithm and Elliptic Curve Digital Signature Algorithm (ECDSA)utilizing a P-512 curve key, as is known in the art. Other algorithmsare known in the art. The SHA-512 hash 424 of the encrypted message 418is used to generate an ECDSA P-512 digital signature 426. The signature426 and encrypted message 418 are stored in the non-volatile memory 201.The image forming device 100 may use the array of numbers 314 in theencrypted message 418 to verify the authenticity of the magnetic key202, and the image forming device 100 may use the digital signature 426to verify the authenticity of the array of numbers 314. In this way, theimage forming device 100 may verify the authenticity of the toner bottle200.

FIG. 5 shows a top view of a magnetic plate 510. The magnetic key 202has a plurality of magnetic plates. The magnetic plate 510 is a diskwith a flat side surface 512. Alternatively, the magnetic plate top viewmay be other shapes including square, triangle, rectangle, arbitraryoutline, etc. Preferably, the magnetic plate 510 has a longest dimensionthat is less than one millimeter so that tightly spaced magnetic platesgenerate a complicated and dense magnetic field structure that isdifficult to counterfeit. Preferably, the magnetic plate 510 has alongest dimension that is more than 0.3 mm to make it easier to locatethe magnetic plate using automatic pick and place equipment e.g. pickand place equipment designed for 0402 SMT components. The flat surface512 is a feature for denoting an orientation which may be any uniqueshape.

FIG. 6. shows a side view of the magnetic plate 510. The magnetic plate510 has a flat top surface 610 and a flat bottom surface 612.Alternatively, the magnetic plate may have a flat bottom surface 612 anda non-flat top surface. The flat bottom surface 612 is useful foruniformly arranging multiple magnetic plates on a flat substrate. Thetop surface 610 is parallel to the flat bottom surface 612 to make iteasier to stack multiple magnetic plates.

The magnetic plate 510 contains, for example, a non-magnetic carrier anda magnetized material. An example non-magnetic carrier is a polymer.Preferably, the polymer is a dielectric polymer such as, for example,acrylic. Preferably, the magnetized material has a magnetic relativepermeability less than two so that magnetic plates may be placed closeto each other and the resulting magnetic field will be approximately thesuperposition of fields of each individual magnetic plate. An examplemagnetized material is neodymium-iron-boron. Other magnetized materialsmay be used. The non-magnetic carrier and the magnetized material may bemixed. Preferably, they would be mixed fifty percent dielectric andfifty percent neodymium-iron-boron by volume to have good mechanicalstrength and good magnetic field strength. Preferably, the non-magneticcarrier has a magnetic relative permeability less than four, and themagnetized material has a magnetic relative permeability less than two.Preferably, the magnetic material has a high initial relative magneticpermeability, e.g. greater than fifty, in a non-magnetized state and alow relative magnetic permeability, e.g. less than four, in a magnetizedstate such as, for example, Neodymium-Praseodymium-iron-Boron alloy.

FIG. 7 shows an alternate magnetic plate geometry that has a core 710surrounded by a region 712 of non-magnetic carrier that surrounds thecore 710 along the plane of the top surface. The core 710 may containmagnetic material or a mixture of magnetic material and non-magneticcarrier. This plate geometry may be lower cost since the amount ofmagnetic material is minimized. The region 712 of non-magnetic carrierprotects the core 710 and makes the magnetic plate easier to pick andplace. The outer shape and the core may be arbitrarily shaped.

FIG. 8 shows a side view of a plurality of magnetic plates with magneticpoles. Magnetic plate 810 has a north pole 812, i.e. the region ofmagnetic plate 810 from which lines of induction diverge, on its topsurface 814 and a south pole 816, i.e. the region of magnetic plate 810from which lines of induction converge, on its bottom surface 818. Astraight line 817 passing through the center of north pole 812 and thecenter of south pole 816 is herein referred to as a magnetic pole line.The orientation of the magnetic pole line 817 is set by the orientationof the magnetic field used to magnetize the magnetic plate 810 relativeto the top surface 814 and bottom surface 818 as is known in the art.The magnetic pole line 817 is orthogonal to the top surface 814.

Magnetic plate 820 has a north pole 822 on its top surface 824 and asouth pole 826 on its bottom surface 828. A magnetic pole line 827 goesthrough the north pole 822 and the south pole 826. Magnetic pole line827 goes through the top surface 824 at an angle, e.g. a forty-fivedegree angle. Magnetic plate 830 has a north pole 832, a south pole 836,and a magnetic pole line 837 that goes through the north pole 832 andthe south pole 836. The magnetic pole line 837 is parallel to the topsurface 824 and the bottom surface 838. Magnetic plate 840 has a northpole 842 on its bottom surface 848 and a south pole 846 on its topsurface 844. A magnetic pole line 847 goes through the north pole 842and the south pole 846, and is at an angle, e.g. a forty-five degreeangle, to the bottom surface 848. Magnetic plate 850 has a north pole852 on its bottom surface 858 and a south pole 856 on its top surface854. A magnetic pole line 857 goes through the north pole 852 and thesouth pole 856 and is orthogonal to the bottom surface 858. Thesemagnetic plates may be arranged in stacked layers. The resultingmagnetic field will be approximately the superposition of each magneticplate's magnetic field. For example, a magnetic key having a layer thatalternates between magnetic plates like magnetic plate 810 and likemagnetic plate 850 will have a magnetic field above a top surface of themagnetic key having an intensity that varies in polarity along the topsurface. Arranging magnetic plates may create other, more complicated,magnetic fields. This example shows five magnetic pole orientations.More or fewer magnetic pole orientations may be used giving finer orcoarser control of the magnetic key field, respectively. The magneticpoles may be the same strength. Alternatively, the magnetic poles maydiffer in strength. The superposition effect works well when therelative permeability is close to one which is the case for magneticmaterials that are highly magnetized and have a high magneticcoercivity. If the material is not saturated, the permeability may bemuch higher than one causing the layered material to distort themagnetic field lines.

FIG. 9 shows a top view of a plurality of magnetic plates 910 a-910 i.These magnetic plates may be located within a magnetic key. In thisexample, each magnetic plate has the same magnetic pole orientation aspreviously described magnetic plate 820 i.e. north pole on its topsurface and magnetic pole line extending through the top surface at anangle. The magnetic plates are arranged in rows, e.g. magnetic plate 910a, 910 b, and 910 c are in a first row, magnetic plate 910 d, 910 e, and910 f are in a second row, and magnetic plate 910 g, 910 h, and 910 iare in a third row forming a two-dimensional grid. The magnetic plateshave the same shape. Some magnetic plates are rotated relative to eachother, e.g. 910 a is rotated ninety degrees relative to 910 b. A morecomplicated magnetic field may be generated above the magnetic key byselectively rotating magnetic plates.

FIG. 10 shows a top view of a plurality of magnetic plates 1010 a-1010h. These plates may be located within a magnetic key. The magneticplates are arranged in rows, e.g. magnetic plate 1010 a, 1010 b, and1010 c are in a first row, magnetic plate 1010 d and 1010 e are in asecond row, and magnetic plate 1010 f, 1010 g, and 1010 h are in a thirdrow. The magnetic plates in the second row are offset from the magneticplates in the first row and the second row. This offset is preferablesince round magnetic plates may be more closely located and thus astronger magnetic field may be generated. Stronger magnetic fields maybe more reliably detected. The closer placed plates also allow for amore complex field pattern making it more difficult to replicate byother means.

FIG. 11 shows a top view of a plurality of magnetic plates 1110 a-1110i. These plates may be located within a magnetic key. The magneticplates are arranged in rows, e.g. magnetic plate 1110 a, 1110 b, and1110 c are in a first row, magnetic plate 1110 d, 1110 e, and 1110 f arein a second row, and magnetic plate 1110 g, 1110 h, 1110 i are in athird row. Magnetic plates 1110 a, 1110 d, 1110 e, 1110 f, and 1110 ihave a north pole on their top surface. Magnetic plates 1110 b, 1110 c,1110 g, and 1110 h have a south pole on their top surface. The magneticplates generate a magnetic field above the top surface of the magnetickey having an intensity that varies in polarity along the top surface. Alow-cost magnetic field sensor may reliably detect this varyingpolarity.

FIG. 12 shows a side view of a magnetic key 1200 having a plurality ofmagnetic plates 1210 a-1210 g. Magnetic plate 1210 a and 1210 c are in afirst layer, magnetic plate 1210 d and 1210 f are in a second layer, andmagnetic plate 1210 e and 1210 g are in a third layer. Magnetic plate1210 b is in both the second layer and the third layer, and is more thantwice as thick as magnetic plate 1210 d. The extra thickness maygenerate a stronger magnetic field. The magnetic plates are stacked incolumns with the bottom surfaces of the magnetic plates in the firstlayer above the top surfaces of the magnetic plates in the second layer.The magnetic plates in the second layer are arranged as shown in FIG. 9.Thus, the plurality of magnetic plates contains at least three magneticpole lines that are not parallel to each other. This results in acomplicated magnetic field above an outer surface 1212 of the magnetickey having an intensity that varies along the outer surface 1212. Thiscomplicated magnetic field is difficult to counterfeit.

Layers of non-magnetic carrier 1214, 1216, may separate the magneticplates. The layers of non-magnetic carrier may contain adhesive.Alternatively, the magnetic plates may be held in place by non-magneticsubstrate material 1218 such as, for example, a UV cured epoxy. Themagnetic plates may be adhered to a non-magnetic substrate 1220 such as,for example, plastic, by an adhesive.

FIG. 13 shows a side view of a magnetic key having a plurality ofmagnetic plates 1310A-1310H. Magnetic plate 1310A, 1310B, and 1310C arein a first layer, magnetic plate 1310D and 1310E are in a second layer,and magnetic plate 1310F, 1310G, and 1310H are in a third layer. Thesecond layer is offset from the first and second layer, resulting in amore complicated magnetic field above an outer surface 1312, which isdifficult to counterfeit. The magnetic plates have flat top surfaces andflat bottom surfaces so the magnetic plates do not collide when thelayers are offset. The layers may be separated by adhesive 1314, 1316.

FIG. 14 shows a top view of a magnet 1400 having magnetic material 1410and a plurality of holes 1412A-1412F extending through the magnet. Themagnetic material 1410 may be Neodymium-Iron-Boron, a mixture of apolymer and Neodymium-Iron-Boron powder, etc. The holes may be laserablated, die cut, etc. Laser ablation is preferred because it createsholes with well-controlled features of arbitrary shape. The magneticmaterial may have a uniform direction of magnetization, a spatiallyvarying direction of magnetization such as, for example, sinusoidal,that varies in polarity along a top surface i.e. the magnetic materialmay have one or more north poles and south poles on its top surface. Theuniform magnetization direction may be perpendicular to the top surface,parallel to the top surface, at an angle relative to the top surface,etc. The holes may contain non-magnetic material, e.g. plastic, air,etc. Preferably, the magnet layer 1400 has a neodymium-iron-boroncompound with a density that is greater than three g/cm³ so that themagnet layer generates a somewhat uniform magnetic field in the regionsaway from holes.

FIG. 15 shows a side view of a magnetic key 1500 having a first magnetlayer 1510 stacked above a second magnet layer 1520 stacked above athird magnet layer 1530. Each layer has holes extending through thelayer as shown in FIG. 14. The first magnet layer 1510 has a magneticpole line 1540 with the north pole on its top surface 1542, the secondmagnet layer 1520 has a magnetic pole line 1550 that is not parallel tomagnetic pole line 1540, and the third magnet layer 1530 has a magneticpole line 1560 with the north pole on its bottom surface 1532. Magnetlayer 1510 and magnet layer 1520 are opposite in polarity to magnetlayer 1530 because of the locations of their north poles. The size,shape, and location of holes in the magnet layers creates a complicatedmagnetic field above the top surface 1570 of the magnetic key due to thesuperposition of the magnetic fields of each layer. The magnet layersmay be separated by layers of adhesive 1580, 1582. Preferably, eachmagnet layer is continuous i.e. the magnet layer surrounds each hole.Preferably, the average hole area of the first magnet layer 1510 is lessthan one square mm, the thickness of each magnet layer is in the rangeof 0.1-0.5 mm, with the total thickness 1590 less than 1.5 mm.

FIG. 16 shows a top view of a magnetic key 1600 having a first magnetlayer 1610 stacked above a second magnet layer. The first magnet layer1610 has a first square hole 1612 and a second square hole 1616. Thesecond magnet layer has a first round hole 1614 and a second round hole1618. Round hole 1614 does not overlap with any holes in the first layerand thus is shown with hidden dashed lines. Round hole 1618 partiallyoverlaps with square hole 1616 and thus part of round hole 1618 is shownwith hidden dashed lines. Triangular hole 1620 overlaps completely onboth the first magnet layer 1610 and the second magnet layer. Partiallyoverlapping holes, and holes that do not overlap, create a more complexmagnetic field than magnetic keys that only have completely overlappingholes.

FIG. 17 shows a top view of a magnetic key 1700 having a first magnetlayer 1710 stacked above a second magnet layer. The first magnet layer1710 has a square hole 1712 and a second square hole 1714. The secondmagnet layer has a square hole 1720 and a second square hole 1722. Theholes on the second layer are larger than the holes on the first magnetlayer 1710. This is preferable, since the holes on the second layer arefarther away from the top surface of the magnetic key 1700 and themagnetic field disturbances from the holes falls off with distance. Forexample, if the average hole area of the holes in the first magnet layerare less than one mm, and the total thickness of the magnet layers istwo mm, it is preferable for the average hole area of the holes in thesecond magnet layer to be greater than one mm.

FIG. 18 shows a top view of a magnetic key 1800 having a first magnetlayer 1810. Holes 1820, 1822, and 1824 are relatively larger than thestrips 1830, 1832 between the holes. These strips form a lattice patternof magnetic material between the holes. The strips generate acomplicated magnetic field above the magnetic key 1800 with sharpvariations over the strips. Preferably, the holes are rectangular suchthat the strips between the holes are rectangular to reduce variabilityin the magnetic field along the length of the strips so that reading themagnetic key is more tolerant to position errors of the reader. However,the holes may be non-rectangular, e.g. circular, triangular, etc.,resulting in non-rectangular strips between the holes. Preferably, eachstrip is less than one mm at its narrowest width to generate a magneticfield that is difficult to counterfeit.

The magnetic keys have magnetic fields that are determined by theproperties and placement of magnetic plates. These are well controlled,and thus the resulting magnetic fields have sufficient strength to bereliably read and sufficient complexity to be difficult to counterfeit.

The foregoing description illustrates various aspects and examples ofthe present disclosure. It is not intended to be exhaustive. Rather, itis chosen to illustrate the principles of the present disclosure and itspractical application to enable one of ordinary skill in the art toutilize the present disclosure, including its various modifications thatnaturally follow. All modifications and variations are contemplatedwithin the scope of the present disclosure as determined by the appendedclaims. Relatively apparent modifications include combining one or morefeatures of various embodiments with features of other embodiments.

What is claimed is:
 1. A supply item for an imaging device comprising: abody; a magnetic key located on the body having a first magnet layerstacked above a second magnet layer, the first magnet layer has a firstplurality of holes extending through the first magnet layer, the secondmagnet layer has a second plurality of holes extending through thesecond magnet layer, the first magnet layer and the second magnet layergenerate a magnetic field having an intensity that varies along a topsurface of the magnetic key; and a non-volatile memory located on thebody containing an array of numbers corresponding to the intensity ofthe magnetic field at a first plurality of locations above the topsurface of the magnetic key and also containing a digital signaturegenerated from the array of numbers, wherein the first magnet layersurrounds the first plurality of holes and the second magnet layersurrounds the second plurality of holes.
 2. The supply item of claim 1,wherein the first magnet layer has a top surface with a north pole onits top surface and the second magnet layer has a top surface with asouth pole on its top surface.
 3. The supply item of claim 1, furthercomprising a third magnet layer stacked below the second magnet layer,the third magnet layer has a third plurality of holes extending throughthe third magnet layer.
 4. The supply item of claim 1, wherein someholes in the first plurality of holes do not overlap with any holes inthe second plurality of holes.
 5. The supply item of claim 1, whereinsome holes in the first plurality of holes partially overlap with someholes in the second plurality of holes.
 6. The supply item of claim 1,wherein the first magnet layer has a top surface with both a north poleand a south pole.
 7. The supply item of claim 1, wherein the firstplurality of holes has an average hole area that is less than one squaremm.
 8. The supply item of claim 7, wherein the first magnet layer has athickness that is between 0.1 and 0.5 mm inclusive.
 9. The supply itemof claim 8, wherein the first magnet layer has a top surface, the secondmagnet layer has a bottom surface, and the distance between the firstmagnet layer top surface and the second magnet layer bottom surface isless than 1.5 mm.
 10. The supply item of claim 1, wherein the firstplurality of holes has a first average hole area, the second pluralityof holes has a second average hole area, and the second average holearea is greater than the first average hole area.
 11. The supply item ofclaim 1, wherein the first magnet layer contains a neodymium-iron-boroncompound having a density greater than three g/cm³.
 12. The supply itemof claim 1, wherein the first plurality of holes was formed by laserablation.
 13. The supply item of claim 1, wherein the magnetic key has abottom surface opposite the top surface of the magnetic key and thearray of numbers contains numbers corresponding to the intensity of themagnetic field at a second plurality of locations below the bottomsurface of the magnetic key.
 14. A supply item for an imaging devicecomprising: a body; a magnetic key located on the body having a magnetlayer having a first hole and a second hole with a strip of the magnetlayer separating the first hole and the second hole, the magnet layergenerates a magnetic field above a top surface of the magnetic keyhaving an intensity that varies along the top surface; and anon-volatile memory located on the body containing an array of numberscorresponding to the intensity of the magnetic field at a plurality oflocations above the top surface and also containing a digital signaturegenerated from the array of numbers, wherein the magnet layer surroundsthe first hole and the second hole, and the first hole and the secondhole are relatively larger than the strip of the magnet layer separatingthe first hole and the second hole.
 15. The supply item of claim 14,wherein the strip of the magnet layer separating the first hole and thesecond hole is rectangular.
 16. The supply item of claim 14, wherein thestrip of the magnet layer separating the first hole and the second holeis less than one mm at its narrowest width.
 17. The supply item of claim14, wherein the magnet layer contains a neodymium-iron-boron compoundhaving a density greater than three g/cm³.
 18. The supply item of claim14, wherein the first hole and the second hole were formed by laserablation.
 19. A supply item for an imaging device comprising: a body; amagnetic key located on the body having a first magnet layer stackedabove a second magnet layer, the first magnet layer has a first holeextending through the first magnet layer, the second magnet layer has asecond hole extending through the second magnet layer, the first magnetlayer and the second magnet layer generate a magnetic field above a topsurface of the magnetic key having an intensity that varies along thetop surface; and a non-volatile memory located on the body containing anarray of numbers corresponding to the intensity of the magnetic field ata plurality of locations above the top surface and also containing adigital signature generated from the array of numbers, wherein the firsthole is positioned directly above the second hole and the second hole islarger than the first hole.
 20. The supply item of claim 19, wherein thefirst hole was formed by laser ablation.